NAME
    Mail::Field - Base class for manipulation of mail header fields

SYNOPSIS
        use Mail::Field;
    
        $field = Mail::Field->new('Subject', 'some subject text');
        print $field->tag,": ",$field->stringify,"\n";

        $field = Mail::Field->subject('some subject text');

DESCRIPTION
    "Mail::Field" is a base class for packages that create and manipulate
    fields from Email (and MIME) headers. Each different field will have its
    own sub-class, defining its own interface.

    This document describes the minimum interface that each sub-class should
    provide, and also guidlines on how the field specific interface should
    be defined.

CONSTRUCTOR
    Mail::Field, and it's sub-classes define several methods which return
    new objects. These can all be termed to be constructors.

    new ( TAG [, STRING | OPTIONS ] )
        The new constructor will create an object in the class which defines
        the field specified by the tag argument.

        After creation of the object :-

        If the tag argument is followed by a single string then the "parse"
        method will be called with this string.

        If the tag argument is followed by more than one arguments then the
        "create" method will be called with these arguments.

    extract ( TAG, HEAD [, INDEX ] )
        This constuctor takes as arguments the tag name, a "Mail::Head"
        object and optionally an index.

        If the index argument is given then "extract" will retrieve the
        given tag from the "Mail::Head" object and create a new
        "Mail::Field" based object. *undef* will be returned in the field
        does not exist.

        If the index argument is not given the the result depends on the
        context in which "extract" is called. If called in a scalar context
        the result will be as if "extract" was called with an index value of
        zero. If called in an array context then all tags will be retrieved
        and a list of "Mail::Field" objects will be returned.

    combine ( FIELD_LIST )
        This constructor takes as arguments a list of "Mail::Field" objects,
        which should all be of the same sub-class, and creates a new object
        in that same class.

        This constructor is nor defined in "Mail::Field" as there is no
        generic way to combine the various field types. Each sub-class
        should define its own combine constructor, if combining is
        possible/allowed.

METHODS
    parse
    set
    tag
    stringify

SUB-CLASS PACKAGE NAMES
    All sub-classes should be called Mail::Field::*name* where *name* is
    derived from the tag using these rules.

    *   Consider a tag as being made up of elements separated by '-'

    *   Convert all characters to lowercase except the first in each
        element, which should be uppercase.

    *   *name* is then created from these elements by using the first N
        characters from each element.

    *   N is calculated by using the formula :-

            int((7 + #elements) / #elements)

    *   *name* is then limited to a maximum of 8 characters, keeping the
        first 8 characters

    For an example of this take a look at the definition of the
    "_header_pkg_name" subroutine in "Mail::Field"

AUTHOR
    Graham Barr <gbarr@pobox.com>

SEE ALSO
    MIME::*s

CREDITS
    Eryq <eryq@rhine.gsfc.nasa.gov> - for all the help in defining this
    package so that Mail::* and MIME::* can be integrated together.

COPYRIGHT
    Copyright (c) 1995-2000 Graham Barr. All rights reserved. This program
    is free software; you can redistribute it and/or modify it under the
    same terms as Perl itself.